Nuclear magnetic resonance (NMR) is the name given to a physical resonance phenomenon involving the observation of specific quantum mechanical magnetic properties of an atomic nucleus in the presence of an applied, external magnetic field. Many scientific techniques exploit NMR phenomena to study molecular physics, crystals and non-crystalline materials through NMR spectroscopy. NMR is also routinely used in advanced medical imaging techniques, such as in magnetic resonance imaging (MRI).
A superconducting quantum interference device (SQUID) is a sensitive detector which is used to measure extremely weak signals, such as subtle changes in the human body's electromagnetic energy field based on the quantum mechanical Josephon effect. A Josephson junction is made up of two superconductors, separated by an insulating layer so thin that electrons can tunnel through. A SQUID consists of tiny loops of superconductors employing Josephson junctions to achieve superposition: each electron moves simultaneously in both directions. Because the current is moving in two opposite directions, the electrons have the ability to perform as qubits (that theoretically could be used to enable quantum computing). SQUIDs have been used for a variety of testing purposes that demand extreme sensitivity, including engineering, medical, and geological equipment.
Both the low field NMR and MRI are based on SQUID, which can avoid the drawbacks of high-field NMR and MRI such as susceptibility artifacts, the cost issue, the size and complexity of the high-field system and so on. The demand of the field homogeneity is not as strict as that of high field NMR/MRI although the signal-to-noise ratio (SNR) is weak in low field NMR/MRI. A homogeneity of 1 part per 104 in the magnetic field can reach a line width of 0.426 Hz in the NMR spectrum. Therefore, the construction of a low field spectrometer of high spectral resolution is much easier than that of the high field NMR/MRI.
The detection sensitivity of NMR/MRI using SQUID designed to image small samples was reported in reference, e.g. K. Schlenga et al., “Low-field magnetic resonance imaging with a high-Tc dc superconducting quantum interference device,” Appl. Phys. Lett. 75, 3695 (1999). For most studies, un-tuned SQUID were used and the samples were mounted under the cryostat with the distance between the sample and detector kept as close as possible because the signal can decay quickly when the distance between sample and detector is increased. It has been demonstrated that the sensitivity decreases rapidly as the separation between the SQUID and the sample increases beyond a certain value. A simple calculation shows that the signal will be reduced by a factor of two as the distance from the SQUID is varied from 1 mm to 50 mm, see S. H. Liao et al., “Enhancement in low field nuclear magnetic resonance with a high-Tc superconducting quantum interference device and hyperpolarized 3He,” J. Appl. Phys. 104, 063918 (2008). However, there are many circumstances in which it is impratical to keep the samples close to the detector.
U.S. Pat. No. 7,218,104 disclosed a method and an apparatus for the detection of NMR signals and production of MRI by obtaining NMR spectra of liquids in microtesla field using prepolarization in millitesla fields and detection with an untuned dc low critical temperature (low-Tc) SQUID. Because the sensitivity of the SQUID is frequency independent, both SNR and spectra resolution are enhanced by detecting the NMR signal in extremely low magnetic fields, where the NMR lines become very narrow for grossly inhomogeneous measurement fields. The detector is a SQUID magnetometer designed so that the SQUID detector can be very close to the sample, which is at room temperature. However, the SQUID magnetometer is so sensitive that when applying magnetization field or RF pulse to the sample, it may affect the SQUID.